Acute Inhibition of Glucose-6-Phosphate Translocator Activity Leads to Increased De Novo Lipogenesis and Development of Hepatic Steatosis Without Affecting VLDL Production in Rats Robert H.J. Bandsma, 1 Coen H. Wiegman, 1 Andreas W. Herling, 2 Hans-Joerg Burger, 2 Anke ter Harmsel, 1 Alfred J. Meijer, 3 Johannes A. Romijn, 4 Dirk-Jan Reijngoud, 1 and Folkert Kuipers 1 Glucose-6-phosphatase (G6Pase) is a key enzyme in hepatic glucose metabolism. Altered G6Pase activity in glycogen storage disease and diabetic states is associ- ated with disturbances in lipid metabolism. We studied the effects of acute inhibition of G6Pase activity on hepatic lipid metabolism in nonanesthetized rats. Rats were infused with an inhibitor of the glucose-6-phos- phate (G6P) translocator (S4048, 30 mg  kg –1  h –1 ) for 8 h. Simultaneously, [1- 13 C]acetate was administered for determination of de novo lipogenesis and fractional cholesterol synthesis rates by mass isotopomer distri- bution analysis. In a separate group of rats, Triton WR 1339 was injected for determination of hepatic VLDL- triglyceride production. S4048 infusion significantly de- creased plasma glucose (11%) and insulin (48%) levels and increased hepatic G6P (201%) and glycogen (182%) contents. Hepatic triglyceride contents in- creased from 5.8  1.4 mol/g liver in controls to 20.6  5.5 mol/g liver in S4048-treated animals. De novo lipogenesis was increased >10-fold in S4048-treated rats, without changes in cholesterol synthesis rates. Hepatic mRNA levels of acetyl-CoA carboxylase and fatty acid synthase were markedly induced. Plasma triglyceride levels increased fourfold, but no differences in plasma cholesterol levels were seen. Surprisingly, hepatic VLDL-triglyceride secretion was not increased in S4048-treated rats. These studies demonstrate that inhibition of the G6Pase system leads to acute stimula- tion of fat synthesis and development of hepatic steato- sis, without affecting hepatic cholesterol synthesis and VLDL secretion. The results emphasize the strong inter- actions that exist between hepatic carbohydrate and fat metabolism. Diabetes 50:2591–2597, 2001 P hosphorylation and dephosporylation of glucose by glucokinase and glucose-6-phosphatase (G6Pase), respectively, are key steps in hepatic glucose uptake and release. The balance between the activities of these enzymes represents an important site for the control of hepatic glucose production (1,2). G6Pase is located in the endoplasmic reticulum (ER) of liver, kidney, and, as recently shown, intestinal cells (3). The glucose-6-phosphate (G6P) metabolizing machinery consists of a putative translocator (4,5) that transports G6P from the cytosol into the ER lumen and a catalytic subunit that converts G6P to glucose and inorganic phos- phate (6). The catalytic subunit is localized to the inner ER membrane. Interestingly, there are several indications to suggest that this site of regulation of glucose metabolism is linked to that of hepatic lipid metabolism. G6Pase activity is increased in patients and animal models of diabetes (2,7,8), probably contributing to increased he- patic glucose production in these conditions. Diabetes is generally associated with hyperlipidemia, which has been found to be mainly due to overproduction of VLDL- triglycerides in type 2 diabetes (9 –11). Deficiency of G6Pase activity, the metabolic basis of glycogen storage disease type I (GSD-1), also leads to abnormalities in lipid metabolism, characterized by severe hypertriglyceridemia and hypercholesterolemia (12–15). Glycogen storage dis- ease (GSD) is caused by mutations in the genes encoding either the putative translocator (type non-1a) (4,5) or the catalytic subunit (type 1a) (6,16,17) of the G6Pase system. Overexpression of hepatic glucokinase also leads to hy- perlipidemia in fed rats (18). Brown et al. (19) showed that the phosphorylation process is important for regulation of assembly and secretion of triglyceride-containing VLDLs by hepatocytes. Little is known about the mechanisms underlying the apparent paradox that hyperlipidemia de- From the 1 Groningen University Institute for Drug Exploration, Center for Liver, Digestive and Metabolic Diseases, Department of Pediatrics, Academic Hospital Groningen, Groningen, the Netherlands; 2 Aventis Pharma Germany, Frankfurt, Germany; the 3 Department of Biochemistry, Academic Medical Center, Amsterdam, the Netherlands; and the 4 Department of Endocrinology, Academic Hospital Leiden, Leiden, the Netherlands. Address correspondence and reprint requests to Robert Bandsma, Depart- ment of Pediatrics, Room Y2117, CMCIV, Academic Hospital Groningen, Hanzeplein 1, P.O. Box 30.001, 9700 RB Groningen, the Netherlands. E-mail: r.h.j.bandsma@med.rug.nl. H.-J.B. and A.W.H. are employed by and hold stock in Aventis Pharma Germany. R.H.J.B. and C.H.W contributed equally to this study. Received for publication 7 August 2000 and accepted in revised form 10 August 2001. ACC, acetyl-CoA carboxylase; ALAT, alanine aminotransferase; apoB, apo- lipoprotein B; ASAT, aspartate aminotransferase; CPT-I, carnitine palmitoyl- transferase I; ER, endoplasmic reticulum; FAS, fatty acid synthase; FFA, free fatty acid; GSD, glycogen storage disease; GSD-1, GSD type I; G6P, glucose- 6-phosphate; G6Pase, glucose-6-phosphatase; HMG-CoA, 3-hydroxy-3-methyl- glutaryl-CoA; MIDA, mass isotopomer distribution analysis; MTP, microsomal triglyceride transfer protein; m/z, mass/charge ratio; PBS, phosphate-buffered saline; PCR, polymerase chain reaction; SREBP, sterol regulatory element binding protein. DIABETES, VOL. 50, NOVEMBER 2001 2591 Downloaded from http://diabetesjournals.org/diabetes/article-pdf/50/11/2591/339080/db1101002591.pdf by guest on 14 November 2022